I just followed all the steps indicated in the book.
When I run "$ sudo service motion start" the
webcam turns ON, then I type my Raspberry Pi
IP address followed by the port 8081
(http://192.168.1.131:8081) on the URL (Browser, locally,
no internet) but I get an open file window instead of the
video.

According to the book It should open the browser (http://192.168.1.131:8081 and shows the video from the webcam. But that does not happen, instead It asks me what to do with the file (from http://192.168.1.131:8081) open, save or cancel. I clicked on open but it takes too much time and does not open anything.

I am not sure which file format is used for the URL, but maybe it's a simple video stream over the HTTP protocol?
Did you try to open the URL in VLC for example, instead of the browser? It might be that your browser is not able to handle a video stream.